1. Field of the Invention
The present invention relates generally to a turbine generator and, more particularly, is directed to a pressure relief diaphragm mounted on a turbine generator and having a construction which prevents air leakage into the turbine during turbine operation.
2. Description of the Prior Art
As well known in the art, a turbine generator includes a turbine housing enclosing a plurality of rows of turbine blades mounted on a common shaft. Pressurized steam forced into the housing and between the turbine blades causes the blades and shaft to rotate. The shaft is connected to a generator operable to convert the mechanical energy created as the shaft rotates to electrical energy. A turbine pressure relief diaphragm communicating with a relief opening in the turbine housing is operable to release excess pressure within the turbine housing generated during turbine operation.
The turbine pressure relief diaphragm itself includes a bird cage-shaped safety housing positioned over the relief opening in the turbine housing. The safety housing includes a mounting flange for receiving bolts which secure the safety housing to a mating flange on the turbine housing and surrounding the relief opening. The pressure relief diaphragm also includes a circular plate assembly which covers the relief opening. The circular plate assembly is formed from a smaller outside diameter plate member secured via a circular pattern of bolts to a larger outside diameter, annular lead ring. Upon assembly, the lead ring is disposed between the safety housing mounting flange and the mating flange on the turbine housing which surrounds the relief opening, and the plate member is positioned within the relief opening. The plate assembly is held in fixed position over the relief opening by the same bolts which secure the safety housing to the turbine housing since these bolts also pass through the lead ring.
During normal operation, the interior of the turbine housing is under vacuum and the exterior of the housing is at atmospheric pressure. The circular plate assembly seals the relief opening in the turbine housing to prevent steam from leaking out of the housing. If excess pressure is generated within the interior of the housing, the pressure relief diaphragm plate assembly shears along an arc located where the outside diameter of the plate member abuts the annular lead ring. The plate member and a portion of the lead ring are forced upwardly into the bird cage-shaped housing. When the plate assembly shears, the interior of the turbine housing is exposed to atmospheric pressure, allowing the excess pressure within the housing to vent to the atmosphere.
Even though the turbine pressure relief diaphragm known and used today functions in a satisfactory manner, the construction of the circular plate assembly may allow air to leak into the turbine housing during turbine generator operation. Over time, this air leakage may result in turbine blade stress corrosion cracking and corrosion fatigue.
Consequently, a need exists for a steam turbine which includes an improved pressure relief diaphragm having a construction which reduces the possibility of air leakage into the interior of the turbine during operation.